Solid polymer membrane (PEM) fuel cell power plants create water on the oxidant reactant surface of the membrane electrode assembly, i.e., the cathode, through electrochemical combination of protons and oxygen on catalyst particles. The water thus produced is commonly referred to as "product water". In addition to product water, water may also accumulate at the oxidant reactant surface, i.e. the cathode, due to the "drag" of water molecules (drag water) in the membrane by the protons formed at the anode during the electrochemical oxidation of hydrogen by a phenomenon known as electro-osmosis. If product and drag water is allowed to uncontrollably accumulate on the oxygen reactant surface of the electrolyte membrane, the accumulated water can impede, and can even prevent oxygen from reacting with protons. Uncontrolled accumulation of water as described above will thus prevent completion of the electrochemical fuel cell process, with the result that the performance of the fuel cell will decrease, and will eventually cause the fuel cell to shut down. This product and drag water must therefore be removed from the active area of the cathode.
Several approaches have been considered for dealing with the problem of removing product water from the cell stack active area in a fuel cell power plant. One approach is to evaporate the product water in the reactant gas stream. This approach has a disadvantage in that it requires that the incoming reactant gases be unsaturated so that the product water (and any water dragged from the anode to the cathode) will evaporate into the unsaturated reactant gas stream.
In PEM cells and PEM cell power plants that employ the aforesaid water removal approach, the reactant flow rate must be sufficiently high to ensure that the reactant stream does not become saturated with water vapor within the flow path across the active area of the cell. Otherwise, saturation of the reactant stream in the flow path across the active area will prevent evaporation of the product and drag water and leave liquid water at the electrode flow path interface. This liquid water will prevent reactant access to the active catalyst in the electrode thereby causing an increase in cell polarization, i.e., mass transport polarization, and a decrease in cell performance and power plant efficiency. Another disadvantage with the removal of product and drag water by evaporation through the use of an unsaturated reactant stream is that the solid polymer membrane itself may become dry, particularly at the reactant inlet of a cell. For a solid polymer fuel cell using perfluorinated sulfonic acid membranes, the water content of the membrane is an important component of the polymer structure. Thus drying out or localized loss of water caused by excessively high evaporation rates of product and membrane water at the reactant inlet can result in drying out of the solid polymer with a decrease in proton conductance and ultimately with the development of cracks and/or holes in the polymer membrane. These holes allow the mixing of the hydrogen and oxygen reactants, commonly called "cross over", with a resultant chemical combustion of cross over reactants; loss of electrochemical energy efficiency; and localized heating. Such localized heating can further promote the loss of water from the membrane and further drying out of the solid polymer, which can accelerate reactant cross over.
A second approach for removing product and drag water from the cathode side of the cells involves the entrainment of the product and drag water as liquid droplets in the fully saturated gas stream so as to expel the product and drag water from the cell stack active area of the power plant. This approach involves high flow rates of the reactant gas stream to sweep the product water off the surface of the electrode and through the flow field. These high flow rates require a large air circulation system and may cause a decrease in the utilization of the reactants, i.e., in the fraction of reactant (oxygen) electrochemically reacted to form water. A decrease in the utilization of the reactant gases lowers the overall efficiency of the fuel cell power plant and requires a larger capacity pump and/or blower to move the reactant stream through the flow field in order to entrain the product water. At very high current densities, oxidant utilizations as low as 5% are necessary to remove the product water. Alternatively, the reactant utilization may be maintained at a desired level by recycling the reactant gas through the power plant. Recycling, however, requires an additional blower and a means for removing the entrained product water, both of which result in a parasitic loss in overall efficiency for the solid polymer fuel cell power plant. It will be appreciated that this solution to the problem of dealing with product and drag water is very complex and expensive.
Another solution to the problem of product and drag water at the cathode side of the cells is disclosed in Austrian Patent No. 389,020 which describes an ion-exchange membrane fuel cell stack that utilizes a fine pore water coolant plate assemblage to provide passive cooling and water management control in the cells in a power plant. The Austrian system utilizes a water-saturated fine pore plate assemblage between the cathode side of one cell and the anode side of the adjacent cell to both cool the cells and to prevent reactant cross-over between adjacent cells. The fine pore plate assemblage is used to move product water away from the cathode side of the ionexchange membrane and into the coolant water stream. The preferred directional movement of the product and coolant water is accomplished by forming the water coolant plate assemblage in two parts, one part having a pore size which will ensure that product water formed on the cathode side will be wicked into the fine pore plate and moved by capillarity toward the water coolant passage network which is inside of the coolant plate assemblage. The coolant plate assemblage also includes a second plate which has a finer pore structure than the first plate, and which is operable to wick water out of the water coolant passages and move that water toward the anode by capillarity. The fine pore and finer pore plates in each assemblage are grooved to form the coolant passage network, and are disposed in face-to-face alignment between adjacent cells. The finer pore plate is thinner than the fine pore plate so as to position the water coolant passages in closer proximity with the anodes than with the cathodes.
The aforesaid solution to water management and cell cooling in ion-exchange membrane fuel cell power plants is difficult to achieve due to the quality control requirements of the fine and finer pore plates, and is also expensive because the plate components are not uniform in thickness and pore size.
The Austrian approach requires that the porous body be filled with water at all times. If, at any time, the porous channels should become devoid of water, the reactant gas can also escape from the active area of the cells through the porous body. This will result in a lessening of cell efficiency with possible commingling of reactant fuel and oxygen and uncontrolled combustion. Since the cell porous bodies must carry electrical current, they must be prepared from conductive material, most commonly carbon particles because of cost and weight constraints. In the experience of the present inventors, such fine pore carbon bodies operate satisfactorily for limited periods but, with time, become non-wetting for water or hydrophobic, and unable to prevent gas escape. Carbon and graphite surfaces must therefore be chemically modified to render them hydrophilic.
One type of modification is by formation of carbon oxides on the surface of the carbon particles through chemical or electrochemical oxidation. The formation of hydroxylic or carboxylic acid species on carbon surfaces is well recognized to render the carbon surface hydrophilic because of their polar nature. However, during operation of the cell the surface carbon oxides can be chemically reduced to reform the initial hydrophobic carbon surface. Thus, with time, during extended operation of the cell the porous body may empty of water and permit reactant gas to escape.
U.S. Pat. No. 4,175,165, granted Nov. 20, 1979 to O. J. Adlhart discloses a fuel cell system having ion exchange membranes and bipolar plates. The bipolar plates are treated so as to have their surfaces be rendered hydrophilic. The surface of the bipolar plates described in this patent are made hydrophilic so as to promote the channeling of water to the edges of the fuel cell assembly. In addition, the bipolar plates described in the patent are said to be gas-impermeable and act to separate adjacent cells one from the other. The plates do not contain a continuous network of pores which permit communication of fluids, either gas or liquid, between adjacent cells. The hydrophilic surface of the bipolar plates is wetted by the product and drag water and channels this water, using the gas flow channels, to the edges of the cell where it is adsorbed and collected, for example, by wicks. The material used to render the bipolar plates hydrophilic are high surface area materials or wetting agents. The materials identified in the patent are colloidal silica sols; high surface area alumina; or high surface area silica-alumina. The silica compounds suggested by this patent for rendering the carbon plate hydrophilic will perform satisfactorily for a limited time period, but the product water percolating through the plates will leach the silica compounds out of the plate so that the plate loses its hydrophilic nature. The solubility of the silica compounds is such that the hydrophilic characteristics of the silica-treated plate are lost over time. This solution is therefore not acceptable for extended use in a fuel cell power plant.
Still another solution to the problem of PEM cell plate wettability is disclosed in U.S. Pat. No. 5,840,414, granted Nov. 24, 1998. The solution described in this patent relates to a fine pore carbon plate for use in a fuel cell power plant which plate is rendered more hydrophilic by partially filling the pores of the carbon plate and also coating the walls of the carbon plates with certain metal oxide compounds. This solution to the problem is useful, but it has certain limitations. One limitation is the processing temperatures that must be reached to form the hydrophilic coatings in the plates. The processing temperatures could result in mechanical or chemical instability in the resultant product. Another limitation with the solution described in this patent relates to the degree of wettability of the metal oxide compounds. There are certain other metal compounds that are more wettable than those described in the patent which could be used to increase the hydrophilicity of the PEM cell plates. These other metal compounds do not require excessively high processing temperatures during fabrication of the fine pore plates.